A helix (pl: helixes or helices) is a type of smooth spacecurve, i.e. a curve in three-dimensional space. It has theproperty that the tangent line at any point makes aconstant angle with a fixed line called the axis. Examples ofhelixes are coil springs and the handrails of spiral staircases.
A "filled-in" helix – for example, a spiral ramp – is calleda helicoids. Helices are important in biology, asthe DNA molecule is formed as two intertwined helices,and many proteins have helical substructures, knownas alpha helices. The word helix comes fromthe Greekword ἕλιξ, "twisted, curved".
Helices can be either right-handed or left-handed. Withthe line of sight along the helixs axis, if a clockwisescrewing motion moves the helix away from theobserver, then it is called a right-handed helix; if towardsthe observer then it is a left-handed helix. Handedness is aproperty of the helix, not of the perspective: a right-handed helix cannot be turned or flipped to look like aleft-handed one unless it is viewed in a mirror, and viceversa.Most hardware screw threads are right-handed helices. The alphahelix in biology as well as the A and B forms of DNA are also right-handed helices.
In mechanical engineering, a helix angle is theangle between any helix and an axial line on itsright, circular cylinder or cone. Commonapplications are screws, helical gears, and wormgears.
Helical gearHelical and worm handednessIn helical and worm gears, the helix angledenotes the standard pitch circle unlessotherwise specified.Application of the helixangle typically employs a magnitude rangingfrom 15 to 30 for helical gears, with 45capping the safe operation limit.
Worm gearWorm gears resemble helical gear seats, thedifference being that the shafts of a worm train arealigned perpendicularly. In this case, the helixangle of the worm meshes with the lead angle ofthe worm gear.